<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <meta name="nodejs.org:node-version" content="v14.19.1">
  <title>Performance measurement APIs | Node.js v14.19.1 Documentation</title>
  <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Lato:400,700,400italic&display=fallback">
  <link rel="stylesheet" href="assets/style.css">
  <link rel="stylesheet" href="assets/hljs.css">
  <link rel="canonical" href="https://nodejs.org/api/perf_hooks.html">
</head>
<body class="alt apidoc" id="api-section-perf_hooks">
  <div id="content" class="clearfix">
    <div id="column2" class="interior">
      <div id="intro" class="interior">
        <a href="/" title="Go back to the home page">
          Node.js
        </a>
      </div>
      <ul>
<li><a href="documentation.html" class="nav-documentation">About this documentation</a></li>
<li><a href="synopsis.html" class="nav-synopsis">Usage and example</a></li>
</ul>
<hr class="line">
<ul>
<li><a href="assert.html" class="nav-assert">Assertion testing</a></li>
<li><a href="async_hooks.html" class="nav-async_hooks">Async hooks</a></li>
<li><a href="buffer.html" class="nav-buffer">Buffer</a></li>
<li><a href="addons.html" class="nav-addons">C++ addons</a></li>
<li><a href="n-api.html" class="nav-n-api">C/C++ addons with Node-API</a></li>
<li><a href="embedding.html" class="nav-embedding">C++ embedder API</a></li>
<li><a href="child_process.html" class="nav-child_process">Child processes</a></li>
<li><a href="cluster.html" class="nav-cluster">Cluster</a></li>
<li><a href="cli.html" class="nav-cli">Command-line options</a></li>
<li><a href="console.html" class="nav-console">Console</a></li>
<li><a href="corepack.html" class="nav-corepack">Corepack</a></li>
<li><a href="crypto.html" class="nav-crypto">Crypto</a></li>
<li><a href="debugger.html" class="nav-debugger">Debugger</a></li>
<li><a href="deprecations.html" class="nav-deprecations">Deprecated APIs</a></li>
<li><a href="diagnostics_channel.html" class="nav-diagnostics_channel">Diagnostics Channel</a></li>
<li><a href="dns.html" class="nav-dns">DNS</a></li>
<li><a href="domain.html" class="nav-domain">Domain</a></li>
<li><a href="errors.html" class="nav-errors">Errors</a></li>
<li><a href="events.html" class="nav-events">Events</a></li>
<li><a href="fs.html" class="nav-fs">File system</a></li>
<li><a href="globals.html" class="nav-globals">Globals</a></li>
<li><a href="http.html" class="nav-http">HTTP</a></li>
<li><a href="http2.html" class="nav-http2">HTTP/2</a></li>
<li><a href="https.html" class="nav-https">HTTPS</a></li>
<li><a href="inspector.html" class="nav-inspector">Inspector</a></li>
<li><a href="intl.html" class="nav-intl">Internationalization</a></li>
<li><a href="modules.html" class="nav-modules">Modules: CommonJS modules</a></li>
<li><a href="esm.html" class="nav-esm">Modules: ECMAScript modules</a></li>
<li><a href="module.html" class="nav-module">Modules: <code>module</code> API</a></li>
<li><a href="packages.html" class="nav-packages">Modules: Packages</a></li>
<li><a href="net.html" class="nav-net">Net</a></li>
<li><a href="os.html" class="nav-os">OS</a></li>
<li><a href="path.html" class="nav-path">Path</a></li>
<li><a href="perf_hooks.html" class="nav-perf_hooks active">Performance hooks</a></li>
<li><a href="policy.html" class="nav-policy">Policies</a></li>
<li><a href="process.html" class="nav-process">Process</a></li>
<li><a href="punycode.html" class="nav-punycode">Punycode</a></li>
<li><a href="querystring.html" class="nav-querystring">Query strings</a></li>
<li><a href="readline.html" class="nav-readline">Readline</a></li>
<li><a href="repl.html" class="nav-repl">REPL</a></li>
<li><a href="report.html" class="nav-report">Report</a></li>
<li><a href="stream.html" class="nav-stream">Stream</a></li>
<li><a href="string_decoder.html" class="nav-string_decoder">String decoder</a></li>
<li><a href="timers.html" class="nav-timers">Timers</a></li>
<li><a href="tls.html" class="nav-tls">TLS/SSL</a></li>
<li><a href="tracing.html" class="nav-tracing">Trace events</a></li>
<li><a href="tty.html" class="nav-tty">TTY</a></li>
<li><a href="dgram.html" class="nav-dgram">UDP/datagram</a></li>
<li><a href="url.html" class="nav-url">URL</a></li>
<li><a href="util.html" class="nav-util">Utilities</a></li>
<li><a href="v8.html" class="nav-v8">V8</a></li>
<li><a href="vm.html" class="nav-vm">VM</a></li>
<li><a href="wasi.html" class="nav-wasi">WASI</a></li>
<li><a href="worker_threads.html" class="nav-worker_threads">Worker threads</a></li>
<li><a href="zlib.html" class="nav-zlib">Zlib</a></li>
</ul>
<hr class="line">
<ul>
<li><a href="https://github.com/nodejs/node" class="nav-https-github-com-nodejs-node">Code repository and issue tracker</a></li>
</ul>
    </div>

    <div id="column1" data-id="perf_hooks" class="interior">
      <header>
        <div class="header-container">
          <h1>Node.js v14.19.1 documentation</h1>
          <button class="theme-toggle-btn" id="theme-toggle-btn" title="Toggle dark mode/light mode" aria-label="Toggle dark mode/light mode" hidden>
            <svg xmlns="http://www.w3.org/2000/svg" class="icon dark-icon" height="24" width="24">
              <path fill="none" d="M0 0h24v24H0z" />
              <path d="M11.1 12.08c-2.33-4.51-.5-8.48.53-10.07C6.27 2.2 1.98 6.59 1.98 12c0 .14.02.28.02.42.62-.27 1.29-.42 2-.42 1.66 0 3.18.83 4.1 2.15A4.01 4.01 0 0111 18c0 1.52-.87 2.83-2.12 3.51.98.32 2.03.5 3.11.5 3.5 0 6.58-1.8 8.37-4.52-2.36.23-6.98-.97-9.26-5.41z"/>
              <path d="M7 16h-.18C6.4 14.84 5.3 14 4 14c-1.66 0-3 1.34-3 3s1.34 3 3 3h3c1.1 0 2-.9 2-2s-.9-2-2-2z"/>
            </svg>
            <svg xmlns="http://www.w3.org/2000/svg" class="icon light-icon" height="24" width="24">
              <path d="M0 0h24v24H0z" fill="none" />
              <path d="M6.76 4.84l-1.8-1.79-1.41 1.41 1.79 1.79 1.42-1.41zM4 10.5H1v2h3v-2zm9-9.95h-2V3.5h2V.55zm7.45 3.91l-1.41-1.41-1.79 1.79 1.41 1.41 1.79-1.79zm-3.21 13.7l1.79 1.8 1.41-1.41-1.8-1.79-1.4 1.4zM20 10.5v2h3v-2h-3zm-8-5c-3.31 0-6 2.69-6 6s2.69 6 6 6 6-2.69 6-6-2.69-6-6-6zm-1 16.95h2V19.5h-2v2.95zm-7.45-3.91l1.41 1.41 1.79-1.8-1.41-1.41-1.79 1.8z"/>
            </svg>
          </button>
        </div>
        <div id="gtoc">
          <ul>
            <li>
              <a href="index.html">Index</a>
            </li>
            <li>
              <a href="all.html">View on single page</a>
            </li>
            <li>
              <a href="perf_hooks.json">View as JSON</a>
            </li>
            
    <li class="version-picker">
      <a href="#">View another version <span>&#x25bc;</span></a>
      <ol class="version-picker"><li><a href="https://nodejs.org/docs/latest-v17.x/api/perf_hooks.html">17.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v16.x/api/perf_hooks.html">16.x <b>LTS</b></a></li>
<li><a href="https://nodejs.org/docs/latest-v15.x/api/perf_hooks.html">15.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v14.x/api/perf_hooks.html">14.x <b>LTS</b></a></li>
<li><a href="https://nodejs.org/docs/latest-v13.x/api/perf_hooks.html">13.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v12.x/api/perf_hooks.html">12.x <b>LTS</b></a></li>
<li><a href="https://nodejs.org/docs/latest-v11.x/api/perf_hooks.html">11.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v10.x/api/perf_hooks.html">10.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v9.x/api/perf_hooks.html">9.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v8.x/api/perf_hooks.html">8.x</a></li></ol>
    </li>
  
            <li class="edit_on_github"><a href="https://github.com/nodejs/node/edit/master/doc/api/perf_hooks.md">Edit on GitHub</a></li>
          </ul>
        </div>
        <hr>
      </header>

      <details id="toc" open><summary>Table of contents</summary><ul>
<li><span class="stability_2"><a href="#perf_hooks_performance_measurement_apis">Performance measurement APIs</a></span>
<ul>
<li><a href="#perf_hooks_perf_hooks_performance"><code>perf_hooks.performance</code></a>
<ul>
<li><a href="#perf_hooks_performance_clearmarks_name"><code>performance.clearMarks([name])</code></a></li>
<li><a href="#perf_hooks_performance_eventlooputilization_utilization1_utilization2"><code>performance.eventLoopUtilization([utilization1[, utilization2]])</code></a></li>
<li><a href="#perf_hooks_performance_mark_name"><code>performance.mark([name])</code></a></li>
<li><a href="#perf_hooks_performance_measure_name_startmark_endmark"><code>performance.measure(name[, startMark[, endMark]])</code></a></li>
<li><a href="#perf_hooks_performance_nodetiming"><code>performance.nodeTiming</code></a></li>
<li><a href="#perf_hooks_performance_now"><code>performance.now()</code></a></li>
<li><a href="#perf_hooks_performance_timeorigin"><code>performance.timeOrigin</code></a></li>
<li><a href="#perf_hooks_performance_timerify_fn"><code>performance.timerify(fn)</code></a></li>
</ul>
</li>
<li><a href="#perf_hooks_class_performanceentry">Class: <code>PerformanceEntry</code></a>
<ul>
<li><a href="#perf_hooks_performanceentry_duration"><code>performanceEntry.duration</code></a></li>
<li><a href="#perf_hooks_performanceentry_entrytype"><code>performanceEntry.entryType</code></a></li>
<li><a href="#perf_hooks_performanceentry_flags"><code>performanceEntry.flags</code></a></li>
<li><a href="#perf_hooks_performanceentry_name"><code>performanceEntry.name</code></a></li>
<li><a href="#perf_hooks_performanceentry_kind"><code>performanceEntry.kind</code></a></li>
<li><a href="#perf_hooks_performanceentry_starttime"><code>performanceEntry.startTime</code></a></li>
</ul>
</li>
<li><a href="#perf_hooks_class_performancenodetiming">Class: <code>PerformanceNodeTiming</code></a>
<ul>
<li><a href="#perf_hooks_performancenodetiming_bootstrapcomplete"><code>performanceNodeTiming.bootstrapComplete</code></a></li>
<li><a href="#perf_hooks_performancenodetiming_environment"><code>performanceNodeTiming.environment</code></a></li>
<li><a href="#perf_hooks_performancenodetiming_idletime"><code>performanceNodeTiming.idleTime</code></a></li>
<li><a href="#perf_hooks_performancenodetiming_loopexit"><code>performanceNodeTiming.loopExit</code></a></li>
<li><a href="#perf_hooks_performancenodetiming_loopstart"><code>performanceNodeTiming.loopStart</code></a></li>
<li><a href="#perf_hooks_performancenodetiming_nodestart"><code>performanceNodeTiming.nodeStart</code></a></li>
<li><a href="#perf_hooks_performancenodetiming_v8start"><code>performanceNodeTiming.v8Start</code></a></li>
</ul>
</li>
<li><a href="#perf_hooks_class_perf_hooks_performanceobserver">Class: <code>perf_hooks.PerformanceObserver</code></a>
<ul>
<li><a href="#perf_hooks_new_performanceobserver_callback"><code>new PerformanceObserver(callback)</code></a></li>
<li><a href="#perf_hooks_performanceobserver_disconnect"><code>performanceObserver.disconnect()</code></a></li>
<li><a href="#perf_hooks_performanceobserver_observe_options"><code>performanceObserver.observe(options)</code></a></li>
</ul>
</li>
<li><a href="#perf_hooks_class_performanceobserverentrylist">Class: <code>PerformanceObserverEntryList</code></a>
<ul>
<li><a href="#perf_hooks_performanceobserverentrylist_getentries"><code>performanceObserverEntryList.getEntries()</code></a></li>
<li><a href="#perf_hooks_performanceobserverentrylist_getentriesbyname_name_type"><code>performanceObserverEntryList.getEntriesByName(name[, type])</code></a></li>
<li><a href="#perf_hooks_performanceobserverentrylist_getentriesbytype_type"><code>performanceObserverEntryList.getEntriesByType(type)</code></a></li>
</ul>
</li>
<li><a href="#perf_hooks_perf_hooks_createhistogram_options"><code>perf_hooks.createHistogram([options])</code></a></li>
<li><a href="#perf_hooks_perf_hooks_monitoreventloopdelay_options"><code>perf_hooks.monitorEventLoopDelay([options])</code></a></li>
<li><a href="#perf_hooks_class_histogram">Class: <code>Histogram</code></a>
<ul>
<li><a href="#perf_hooks_histogram_exceeds"><code>histogram.exceeds</code></a></li>
<li><a href="#perf_hooks_histogram_max"><code>histogram.max</code></a></li>
<li><a href="#perf_hooks_histogram_mean"><code>histogram.mean</code></a></li>
<li><a href="#perf_hooks_histogram_min"><code>histogram.min</code></a></li>
<li><a href="#perf_hooks_histogram_percentile_percentile"><code>histogram.percentile(percentile)</code></a></li>
<li><a href="#perf_hooks_histogram_percentiles"><code>histogram.percentiles</code></a></li>
<li><a href="#perf_hooks_histogram_reset"><code>histogram.reset()</code></a></li>
<li><a href="#perf_hooks_histogram_stddev"><code>histogram.stddev</code></a></li>
</ul>
</li>
<li><a href="#perf_hooks_class_intervalhistogram_extends_histogram">Class: <code>IntervalHistogram extends Histogram</code></a>
<ul>
<li><a href="#perf_hooks_histogram_disable"><code>histogram.disable()</code></a></li>
<li><a href="#perf_hooks_histogram_enable"><code>histogram.enable()</code></a></li>
<li><a href="#perf_hooks_cloning_an_intervalhistogram">Cloning an <code>IntervalHistogram</code></a></li>
</ul>
</li>
<li><a href="#perf_hooks_class_recordablehistogram_extends_histogram">Class: <code>RecordableHistogram extends Histogram</code></a>
<ul>
<li><a href="#perf_hooks_histogram_record_val"><code>histogram.record(val)</code></a></li>
<li><a href="#perf_hooks_histogram_recorddelta"><code>histogram.recordDelta()</code></a></li>
</ul>
</li>
<li><a href="#perf_hooks_examples">Examples</a>
<ul>
<li><a href="#perf_hooks_measuring_the_duration_of_async_operations">Measuring the duration of async operations</a></li>
<li><a href="#perf_hooks_measuring_how_long_it_takes_to_load_dependencies">Measuring how long it takes to load dependencies</a></li>
</ul>
</li>
</ul>
</li>
</ul></details>

      <div id="apicontent">
        <h2>Performance measurement APIs<span><a class="mark" href="#perf_hooks_performance_measurement_apis" id="perf_hooks_performance_measurement_apis">#</a></span></h2>

<p></p><div class="api_stability api_stability_2"><a href="documentation.html#documentation_stability_index">Stability: 2</a> - Stable</div><p></p>
<p><strong>Source Code:</strong> <a href="https://github.com/nodejs/node/blob/v14.19.1/lib/perf_hooks.js">lib/perf_hooks.js</a></p>
<p>This module provides an implementation of a subset of the W3C
<a href="https://w3c.github.io/perf-timing-primer/">Web Performance APIs</a> as well as additional APIs for
Node.js-specific performance measurements.</p>
<p>Node.js supports the following <a href="https://w3c.github.io/perf-timing-primer/">Web Performance APIs</a>:</p>
<ul>
<li><a href="https://www.w3.org/TR/hr-time-2">High Resolution Time</a></li>
<li><a href="https://w3c.github.io/performance-timeline/">Performance Timeline</a></li>
<li><a href="https://www.w3.org/TR/user-timing/">User Timing</a></li>
</ul>
<pre><code class="language-js"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">PerformanceObserver</span>, performance } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'perf_hooks'</span>);

<span class="hljs-keyword">const</span> obs = <span class="hljs-keyword">new</span> <span class="hljs-title class_">PerformanceObserver</span>(<span class="hljs-function">(<span class="hljs-params">items</span>) =></span> {
  <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(items.<span class="hljs-title function_">getEntries</span>()[<span class="hljs-number">0</span>].<span class="hljs-property">duration</span>);
  performance.<span class="hljs-title function_">clearMarks</span>();
});
obs.<span class="hljs-title function_">observe</span>({ <span class="hljs-attr">entryTypes</span>: [<span class="hljs-string">'measure'</span>] });
performance.<span class="hljs-title function_">measure</span>(<span class="hljs-string">'Start to Now'</span>);

performance.<span class="hljs-title function_">mark</span>(<span class="hljs-string">'A'</span>);
<span class="hljs-title function_">doSomeLongRunningProcess</span>(<span class="hljs-function">() =></span> {
  performance.<span class="hljs-title function_">measure</span>(<span class="hljs-string">'A to Now'</span>, <span class="hljs-string">'A'</span>);

  performance.<span class="hljs-title function_">mark</span>(<span class="hljs-string">'B'</span>);
  performance.<span class="hljs-title function_">measure</span>(<span class="hljs-string">'A to B'</span>, <span class="hljs-string">'A'</span>, <span class="hljs-string">'B'</span>);
});</code></pre>
<section><h3><code>perf_hooks.performance</code><span><a class="mark" href="#perf_hooks_perf_hooks_performance" id="perf_hooks_perf_hooks_performance">#</a></span></h3>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<p>An object that can be used to collect performance metrics from the current
Node.js instance. It is similar to <a href="https://developer.mozilla.org/en-US/docs/Web/API/Window/performance"><code>window.performance</code></a> in browsers.</p>
<h4><code>performance.clearMarks([name])</code><span><a class="mark" href="#perf_hooks_performance_clearmarks_name" id="perf_hooks_performance_clearmarks_name">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li><code>name</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type">&#x3C;string></a></li>
</ul>
<p>If <code>name</code> is not provided, removes all <code>PerformanceMark</code> objects from the
Performance Timeline. If <code>name</code> is provided, removes only the named mark.</p>
<h4><code>performance.eventLoopUtilization([utilization1[, utilization2]])</code><span><a class="mark" href="#perf_hooks_performance_eventlooputilization_utilization1_utilization2" id="perf_hooks_performance_eventlooputilization_utilization1_utilization2">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v14.10.0, v12.19.0</span>
</div>
<ul>
<li><code>utilization1</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type">&#x3C;Object></a> The result of a previous call to
<code>eventLoopUtilization()</code>.</li>
<li><code>utilization2</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type">&#x3C;Object></a> The result of a previous call to
<code>eventLoopUtilization()</code> prior to <code>utilization1</code>.</li>
<li>Returns <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type">&#x3C;Object></a>
<ul>
<li><code>idle</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
<li><code>active</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
<li><code>utilization</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
</li>
</ul>
<p>The <code>eventLoopUtilization()</code> method returns an object that contains the
cumulative duration of time the event loop has been both idle and active as a
high resolution milliseconds timer. The <code>utilization</code> value is the calculated
Event Loop Utilization (ELU).</p>
<p>If bootstrapping has not yet finished on the main thread the properties have
the value of <code>0</code>. The ELU is immediately available on <a href="worker_threads.html#worker_threads_worker_threads">Worker threads</a> since
bootstrap happens within the event loop.</p>
<p>Both <code>utilization1</code> and <code>utilization2</code> are optional parameters.</p>
<p>If <code>utilization1</code> is passed, then the delta between the current call's <code>active</code>
and <code>idle</code> times, as well as the corresponding <code>utilization</code> value are
calculated and returned (similar to <a href="process.html#process_process_hrtime_time"><code>process.hrtime()</code></a>).</p>
<p>If <code>utilization1</code> and <code>utilization2</code> are both passed, then the delta is
calculated between the two arguments. This is a convenience option because,
unlike <a href="process.html#process_process_hrtime_time"><code>process.hrtime()</code></a>, calculating the ELU is more complex than a
single subtraction.</p>
<p>ELU is similar to CPU utilization, except that it only measures event loop
statistics and not CPU usage. It represents the percentage of time the event
loop has spent outside the event loop's event provider (e.g. <code>epoll_wait</code>).
No other CPU idle time is taken into consideration. The following is an example
of how a mostly idle process will have a high ELU.</p>
<pre><code class="language-js"><span class="hljs-meta">'use strict'</span>;
<span class="hljs-keyword">const</span> { eventLoopUtilization } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'perf_hooks'</span>).<span class="hljs-property">performance</span>;
<span class="hljs-keyword">const</span> { spawnSync } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'child_process'</span>);

<span class="hljs-title function_">setImmediate</span>(<span class="hljs-function">() =></span> {
  <span class="hljs-keyword">const</span> elu = <span class="hljs-title function_">eventLoopUtilization</span>();
  <span class="hljs-title function_">spawnSync</span>(<span class="hljs-string">'sleep'</span>, [<span class="hljs-string">'5'</span>]);
  <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">eventLoopUtilization</span>(elu).<span class="hljs-property">utilization</span>);
});</code></pre>
<p>Although the CPU is mostly idle while running this script, the value of
<code>utilization</code> is <code>1</code>. This is because the call to
<a href="child_process.html#child_process_child_process_spawnsync_command_args_options"><code>child_process.spawnSync()</code></a> blocks the event loop from proceeding.</p>
<p>Passing in a user-defined object instead of the result of a previous call to
<code>eventLoopUtilization()</code> will lead to undefined behavior. The return values
are not guaranteed to reflect any correct state of the event loop.</p>
<h4><code>performance.mark([name])</code><span><a class="mark" href="#perf_hooks_performance_mark_name" id="perf_hooks_performance_mark_name">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li><code>name</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type">&#x3C;string></a></li>
</ul>
<p>Creates a new <code>PerformanceMark</code> entry in the Performance Timeline. A
<code>PerformanceMark</code> is a subclass of <code>PerformanceEntry</code> whose
<code>performanceEntry.entryType</code> is always <code>'mark'</code>, and whose
<code>performanceEntry.duration</code> is always <code>0</code>. Performance marks are used
to mark specific significant moments in the Performance Timeline.</p>
<h4><code>performance.measure(name[, startMark[, endMark]])</code><span><a class="mark" href="#perf_hooks_performance_measure_name_startmark_endmark" id="perf_hooks_performance_measure_name_startmark_endmark">#</a></span></h4>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v14.0.0</td>
<td><p>Make <code>startMark</code> and <code>endMark</code> parameters optional.</p></td></tr>
<tr><td>v8.5.0</td>
<td><p><span>Added in: v8.5.0</span></p></td></tr>
</tbody></table>
</details>
</div>
<ul>
<li><code>name</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type">&#x3C;string></a></li>
<li><code>startMark</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type">&#x3C;string></a> Optional.</li>
<li><code>endMark</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type">&#x3C;string></a> Optional.</li>
</ul>
<p>Creates a new <code>PerformanceMeasure</code> entry in the Performance Timeline. A
<code>PerformanceMeasure</code> is a subclass of <code>PerformanceEntry</code> whose
<code>performanceEntry.entryType</code> is always <code>'measure'</code>, and whose
<code>performanceEntry.duration</code> measures the number of milliseconds elapsed since
<code>startMark</code> and <code>endMark</code>.</p>
<p>The <code>startMark</code> argument may identify any <em>existing</em> <code>PerformanceMark</code> in the
Performance Timeline, or <em>may</em> identify any of the timestamp properties
provided by the <code>PerformanceNodeTiming</code> class. If the named <code>startMark</code> does
not exist, then <code>startMark</code> is set to <a href="https://w3c.github.io/hr-time/#dom-performance-timeorigin"><code>timeOrigin</code></a> by default.</p>
<p>The optional <code>endMark</code> argument must identify any <em>existing</em> <code>PerformanceMark</code>
in the Performance Timeline or any of the timestamp properties provided by the
<code>PerformanceNodeTiming</code> class. <code>endMark</code> will be <code>performance.now()</code>
if no parameter is passed, otherwise if the named <code>endMark</code> does not exist, an
error will be thrown.</p>
<h4><code>performance.nodeTiming</code><span><a class="mark" href="#perf_hooks_performance_nodetiming" id="perf_hooks_performance_nodetiming">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li><a href="perf_hooks.html#perf_hooks_class_performancenodetiming" class="type">&#x3C;PerformanceNodeTiming></a></li>
</ul>
<p><em>This property is an extension by Node.js. It is not available in Web browsers.</em></p>
<p>An instance of the <code>PerformanceNodeTiming</code> class that provides performance
metrics for specific Node.js operational milestones.</p>
<h4><code>performance.now()</code><span><a class="mark" href="#perf_hooks_performance_now" id="perf_hooks_performance_now">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
<p>Returns the current high resolution millisecond timestamp, where 0 represents
the start of the current <code>node</code> process.</p>
<h4><code>performance.timeOrigin</code><span><a class="mark" href="#perf_hooks_performance_timeorigin" id="perf_hooks_performance_timeorigin">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
<p>The <a href="https://w3c.github.io/hr-time/#dom-performance-timeorigin"><code>timeOrigin</code></a> specifies the high resolution millisecond timestamp at
which the current <code>node</code> process began, measured in Unix time.</p>
<h4><code>performance.timerify(fn)</code><span><a class="mark" href="#perf_hooks_performance_timerify_fn" id="perf_hooks_performance_timerify_fn">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li><code>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&#x3C;Function></a></li>
</ul>
<p><em>This property is an extension by Node.js. It is not available in Web browsers.</em></p>
<p>Wraps a function within a new function that measures the running time of the
wrapped function. A <code>PerformanceObserver</code> must be subscribed to the <code>'function'</code>
event type in order for the timing details to be accessed.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> {
  performance,
  <span class="hljs-title class_">PerformanceObserver</span>
} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'perf_hooks'</span>);

<span class="hljs-keyword">function</span> <span class="hljs-title function_">someFunction</span>(<span class="hljs-params"></span>) {
  <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'hello world'</span>);
}

<span class="hljs-keyword">const</span> wrapped = performance.<span class="hljs-title function_">timerify</span>(someFunction);

<span class="hljs-keyword">const</span> obs = <span class="hljs-keyword">new</span> <span class="hljs-title class_">PerformanceObserver</span>(<span class="hljs-function">(<span class="hljs-params">list</span>) =></span> {
  <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(list.<span class="hljs-title function_">getEntries</span>()[<span class="hljs-number">0</span>].<span class="hljs-property">duration</span>);
  obs.<span class="hljs-title function_">disconnect</span>();
});
obs.<span class="hljs-title function_">observe</span>({ <span class="hljs-attr">entryTypes</span>: [<span class="hljs-string">'function'</span>] });

<span class="hljs-comment">// A performance timeline entry will be created</span>
<span class="hljs-title function_">wrapped</span>();</code></pre>
</section><section><h3>Class: <code>PerformanceEntry</code><span><a class="mark" href="#perf_hooks_class_performanceentry" id="perf_hooks_class_performanceentry">#</a></span></h3>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<h4><code>performanceEntry.duration</code><span><a class="mark" href="#perf_hooks_performanceentry_duration" id="perf_hooks_performanceentry_duration">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
<p>The total number of milliseconds elapsed for this entry. This value will not
be meaningful for all Performance Entry types.</p>
<h4><code>performanceEntry.entryType</code><span><a class="mark" href="#perf_hooks_performanceentry_entrytype" id="perf_hooks_performanceentry_entrytype">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type">&#x3C;string></a></li>
</ul>
<p>The type of the performance entry. It may be one of:</p>
<ul>
<li><code>'node'</code> (Node.js only)</li>
<li><code>'mark'</code> (available on the Web)</li>
<li><code>'measure'</code> (available on the Web)</li>
<li><code>'gc'</code> (Node.js only)</li>
<li><code>'function'</code> (Node.js only)</li>
<li><code>'http2'</code> (Node.js only)</li>
<li><code>'http'</code> (Node.js only)</li>
</ul>
<h4><code>performanceEntry.flags</code><span><a class="mark" href="#perf_hooks_performanceentry_flags" id="perf_hooks_performanceentry_flags">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v13.9.0, v12.17.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
<p><em>This property is an extension by Node.js. It is not available in Web browsers.</em></p>
<p>When <code>performanceEntry.entryType</code> is equal to <code>'gc'</code>, the <code>performance.flags</code>
property contains additional information about garbage collection operation.
The value may be one of:</p>
<ul>
<li><code>perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_NO</code></li>
<li><code>perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_CONSTRUCT_RETAINED</code></li>
<li><code>perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_FORCED</code></li>
<li><code>perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_SYNCHRONOUS_PHANTOM_PROCESSING</code></li>
<li><code>perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_ALL_AVAILABLE_GARBAGE</code></li>
<li><code>perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_ALL_EXTERNAL_MEMORY</code></li>
<li><code>perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_SCHEDULE_IDLE</code></li>
</ul>
<h4><code>performanceEntry.name</code><span><a class="mark" href="#perf_hooks_performanceentry_name" id="perf_hooks_performanceentry_name">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type">&#x3C;string></a></li>
</ul>
<p>The name of the performance entry.</p>
<h4><code>performanceEntry.kind</code><span><a class="mark" href="#perf_hooks_performanceentry_kind" id="perf_hooks_performanceentry_kind">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
<p><em>This property is an extension by Node.js. It is not available in Web browsers.</em></p>
<p>When <code>performanceEntry.entryType</code> is equal to <code>'gc'</code>, the <code>performance.kind</code>
property identifies the type of garbage collection operation that occurred.
The value may be one of:</p>
<ul>
<li><code>perf_hooks.constants.NODE_PERFORMANCE_GC_MAJOR</code></li>
<li><code>perf_hooks.constants.NODE_PERFORMANCE_GC_MINOR</code></li>
<li><code>perf_hooks.constants.NODE_PERFORMANCE_GC_INCREMENTAL</code></li>
<li><code>perf_hooks.constants.NODE_PERFORMANCE_GC_WEAKCB</code></li>
</ul>
<h4><code>performanceEntry.startTime</code><span><a class="mark" href="#perf_hooks_performanceentry_starttime" id="perf_hooks_performanceentry_starttime">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
<p>The high resolution millisecond timestamp marking the starting time of the
Performance Entry.</p>
</section><section><h3>Class: <code>PerformanceNodeTiming</code><span><a class="mark" href="#perf_hooks_class_performancenodetiming" id="perf_hooks_class_performancenodetiming">#</a></span></h3>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li>Extends: <a href="perf_hooks.html#perf_hooks_class_performanceentry" class="type">&#x3C;PerformanceEntry></a></li>
</ul>
<p><em>This property is an extension by Node.js. It is not available in Web browsers.</em></p>
<p>Provides timing details for Node.js itself. The constructor of this class
is not exposed to users.</p>
<h4><code>performanceNodeTiming.bootstrapComplete</code><span><a class="mark" href="#perf_hooks_performancenodetiming_bootstrapcomplete" id="perf_hooks_performancenodetiming_bootstrapcomplete">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
<p>The high resolution millisecond timestamp at which the Node.js process
completed bootstrapping. If bootstrapping has not yet finished, the property
has the value of -1.</p>
<h4><code>performanceNodeTiming.environment</code><span><a class="mark" href="#perf_hooks_performancenodetiming_environment" id="perf_hooks_performancenodetiming_environment">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
<p>The high resolution millisecond timestamp at which the Node.js environment was
initialized.</p>
<h4><code>performanceNodeTiming.idleTime</code><span><a class="mark" href="#perf_hooks_performancenodetiming_idletime" id="perf_hooks_performancenodetiming_idletime">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v14.10.0, v12.19.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
<p>The high resolution millisecond timestamp of the amount of time the event loop
has been idle within the event loop's event provider (e.g. <code>epoll_wait</code>). This
does not take CPU usage into consideration. If the event loop has not yet
started (e.g., in the first tick of the main script), the property has the
value of 0.</p>
<h4><code>performanceNodeTiming.loopExit</code><span><a class="mark" href="#perf_hooks_performancenodetiming_loopexit" id="perf_hooks_performancenodetiming_loopexit">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
<p>The high resolution millisecond timestamp at which the Node.js event loop
exited. If the event loop has not yet exited, the property has the value of -1.
It can only have a value of not -1 in a handler of the <a href="process.html#process_event_exit"><code>'exit'</code></a> event.</p>
<h4><code>performanceNodeTiming.loopStart</code><span><a class="mark" href="#perf_hooks_performancenodetiming_loopstart" id="perf_hooks_performancenodetiming_loopstart">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
<p>The high resolution millisecond timestamp at which the Node.js event loop
started. If the event loop has not yet started (e.g., in the first tick of the
main script), the property has the value of -1.</p>
<h4><code>performanceNodeTiming.nodeStart</code><span><a class="mark" href="#perf_hooks_performancenodetiming_nodestart" id="perf_hooks_performancenodetiming_nodestart">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
<p>The high resolution millisecond timestamp at which the Node.js process was
initialized.</p>
<h4><code>performanceNodeTiming.v8Start</code><span><a class="mark" href="#perf_hooks_performancenodetiming_v8start" id="perf_hooks_performancenodetiming_v8start">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
<p>The high resolution millisecond timestamp at which the V8 platform was
initialized.</p>
</section><section><h3>Class: <code>perf_hooks.PerformanceObserver</code><span><a class="mark" href="#perf_hooks_class_perf_hooks_performanceobserver" id="perf_hooks_class_perf_hooks_performanceobserver">#</a></span></h3>
<h4><code>new PerformanceObserver(callback)</code><span><a class="mark" href="#perf_hooks_new_performanceobserver_callback" id="perf_hooks_new_performanceobserver_callback">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li><code>callback</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&#x3C;Function></a>
<ul>
<li><code>list</code> <a href="perf_hooks.html#perf_hooks_class_performanceobserverentrylist" class="type">&#x3C;PerformanceObserverEntryList></a></li>
<li><code>observer</code> <a href="perf_hooks.html#perf_hooks_class_perf_hooks_performanceobserver" class="type">&#x3C;PerformanceObserver></a></li>
</ul>
</li>
</ul>
<p><code>PerformanceObserver</code> objects provide notifications when new
<code>PerformanceEntry</code> instances have been added to the Performance Timeline.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> {
  performance,
  <span class="hljs-title class_">PerformanceObserver</span>
} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'perf_hooks'</span>);

<span class="hljs-keyword">const</span> obs = <span class="hljs-keyword">new</span> <span class="hljs-title class_">PerformanceObserver</span>(<span class="hljs-function">(<span class="hljs-params">list, observer</span>) =></span> {
  <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(list.<span class="hljs-title function_">getEntries</span>());
  observer.<span class="hljs-title function_">disconnect</span>();
});
obs.<span class="hljs-title function_">observe</span>({ <span class="hljs-attr">entryTypes</span>: [<span class="hljs-string">'mark'</span>], <span class="hljs-attr">buffered</span>: <span class="hljs-literal">true</span> });

performance.<span class="hljs-title function_">mark</span>(<span class="hljs-string">'test'</span>);</code></pre>
<p>Because <code>PerformanceObserver</code> instances introduce their own additional
performance overhead, instances should not be left subscribed to notifications
indefinitely. Users should disconnect observers as soon as they are no
longer needed.</p>
<p>The <code>callback</code> is invoked when a <code>PerformanceObserver</code> is
notified about new <code>PerformanceEntry</code> instances. The callback receives a
<code>PerformanceObserverEntryList</code> instance and a reference to the
<code>PerformanceObserver</code>.</p>
<h4><code>performanceObserver.disconnect()</code><span><a class="mark" href="#perf_hooks_performanceobserver_disconnect" id="perf_hooks_performanceobserver_disconnect">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<p>Disconnects the <code>PerformanceObserver</code> instance from all notifications.</p>
<h4><code>performanceObserver.observe(options)</code><span><a class="mark" href="#perf_hooks_performanceobserver_observe_options" id="perf_hooks_performanceobserver_observe_options">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type">&#x3C;Object></a>
<ul>
<li><code>entryTypes</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type">&#x3C;string[]></a> An array of strings identifying the types of
<code>PerformanceEntry</code> instances the observer is interested in. If not
provided an error will be thrown.</li>
<li><code>buffered</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type">&#x3C;boolean></a> If true, the notification callback will be
called using <code>setImmediate()</code> and multiple <code>PerformanceEntry</code> instance
notifications will be buffered internally. If <code>false</code>, notifications will
be immediate and synchronous. <strong>Default:</strong> <code>false</code>.</li>
</ul>
</li>
</ul>
<p>Subscribes the <code>PerformanceObserver</code> instance to notifications of new
<code>PerformanceEntry</code> instances identified by <code>options.entryTypes</code>.</p>
<p>When <code>options.buffered</code> is <code>false</code>, the <code>callback</code> will be invoked once for
every <code>PerformanceEntry</code> instance:</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> {
  performance,
  <span class="hljs-title class_">PerformanceObserver</span>
} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'perf_hooks'</span>);

<span class="hljs-keyword">const</span> obs = <span class="hljs-keyword">new</span> <span class="hljs-title class_">PerformanceObserver</span>(<span class="hljs-function">(<span class="hljs-params">list, observer</span>) =></span> {
  <span class="hljs-comment">// Called three times synchronously. `list` contains one item.</span>
});
obs.<span class="hljs-title function_">observe</span>({ <span class="hljs-attr">entryTypes</span>: [<span class="hljs-string">'mark'</span>] });

<span class="hljs-keyword">for</span> (<span class="hljs-keyword">let</span> n = <span class="hljs-number">0</span>; n &#x3C; <span class="hljs-number">3</span>; n++)
  performance.<span class="hljs-title function_">mark</span>(<span class="hljs-string">`test<span class="hljs-subst">${n}</span>`</span>);</code></pre>
<pre><code class="language-js"><span class="hljs-keyword">const</span> {
  performance,
  <span class="hljs-title class_">PerformanceObserver</span>
} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'perf_hooks'</span>);

<span class="hljs-keyword">const</span> obs = <span class="hljs-keyword">new</span> <span class="hljs-title class_">PerformanceObserver</span>(<span class="hljs-function">(<span class="hljs-params">list, observer</span>) =></span> {
  <span class="hljs-comment">// Called once. `list` contains three items.</span>
});
obs.<span class="hljs-title function_">observe</span>({ <span class="hljs-attr">entryTypes</span>: [<span class="hljs-string">'mark'</span>], <span class="hljs-attr">buffered</span>: <span class="hljs-literal">true</span> });

<span class="hljs-keyword">for</span> (<span class="hljs-keyword">let</span> n = <span class="hljs-number">0</span>; n &#x3C; <span class="hljs-number">3</span>; n++)
  performance.<span class="hljs-title function_">mark</span>(<span class="hljs-string">`test<span class="hljs-subst">${n}</span>`</span>);</code></pre>
</section><section><h3>Class: <code>PerformanceObserverEntryList</code><span><a class="mark" href="#perf_hooks_class_performanceobserverentrylist" id="perf_hooks_class_performanceobserverentrylist">#</a></span></h3>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<p>The <code>PerformanceObserverEntryList</code> class is used to provide access to the
<code>PerformanceEntry</code> instances passed to a <code>PerformanceObserver</code>.
The constructor of this class is not exposed to users.</p>
<h4><code>performanceObserverEntryList.getEntries()</code><span><a class="mark" href="#perf_hooks_performanceobserverentrylist_getentries" id="perf_hooks_performanceobserverentrylist_getentries">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li>Returns: <a href="perf_hooks.html#perf_hooks_class_performanceentry" class="type">&#x3C;PerformanceEntry[]></a></li>
</ul>
<p>Returns a list of <code>PerformanceEntry</code> objects in chronological order
with respect to <code>performanceEntry.startTime</code>.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> {
  performance,
  <span class="hljs-title class_">PerformanceObserver</span>
} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'perf_hooks'</span>);

<span class="hljs-keyword">const</span> obs = <span class="hljs-keyword">new</span> <span class="hljs-title class_">PerformanceObserver</span>(<span class="hljs-function">(<span class="hljs-params">perfObserverList, observer</span>) =></span> {
  <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(perfObserverList.<span class="hljs-title function_">getEntries</span>());
  <span class="hljs-comment">/**
   * [
   *   PerformanceEntry {
   *     name: 'test',
   *     entryType: 'mark',
   *     startTime: 81.465639,
   *     duration: 0
   *   },
   *   PerformanceEntry {
   *     name: 'meow',
   *     entryType: 'mark',
   *     startTime: 81.860064,
   *     duration: 0
   *   }
   * ]
   */</span>
  observer.<span class="hljs-title function_">disconnect</span>();
});
obs.<span class="hljs-title function_">observe</span>({ <span class="hljs-attr">entryTypes</span>: [<span class="hljs-string">'mark'</span>], <span class="hljs-attr">buffered</span>: <span class="hljs-literal">true</span> });

performance.<span class="hljs-title function_">mark</span>(<span class="hljs-string">'test'</span>);
performance.<span class="hljs-title function_">mark</span>(<span class="hljs-string">'meow'</span>);</code></pre>
<h4><code>performanceObserverEntryList.getEntriesByName(name[, type])</code><span><a class="mark" href="#perf_hooks_performanceobserverentrylist_getentriesbyname_name_type" id="perf_hooks_performanceobserverentrylist_getentriesbyname_name_type">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li><code>name</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type">&#x3C;string></a></li>
<li><code>type</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type">&#x3C;string></a></li>
<li>Returns: <a href="perf_hooks.html#perf_hooks_class_performanceentry" class="type">&#x3C;PerformanceEntry[]></a></li>
</ul>
<p>Returns a list of <code>PerformanceEntry</code> objects in chronological order
with respect to <code>performanceEntry.startTime</code> whose <code>performanceEntry.name</code> is
equal to <code>name</code>, and optionally, whose <code>performanceEntry.entryType</code> is equal to
<code>type</code>.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> {
  performance,
  <span class="hljs-title class_">PerformanceObserver</span>
} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'perf_hooks'</span>);

<span class="hljs-keyword">const</span> obs = <span class="hljs-keyword">new</span> <span class="hljs-title class_">PerformanceObserver</span>(<span class="hljs-function">(<span class="hljs-params">perfObserverList, observer</span>) =></span> {
  <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(perfObserverList.<span class="hljs-title function_">getEntriesByName</span>(<span class="hljs-string">'meow'</span>));
  <span class="hljs-comment">/**
   * [
   *   PerformanceEntry {
   *     name: 'meow',
   *     entryType: 'mark',
   *     startTime: 98.545991,
   *     duration: 0
   *   }
   * ]
   */</span>
  <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(perfObserverList.<span class="hljs-title function_">getEntriesByName</span>(<span class="hljs-string">'nope'</span>)); <span class="hljs-comment">// []</span>

  <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(perfObserverList.<span class="hljs-title function_">getEntriesByName</span>(<span class="hljs-string">'test'</span>, <span class="hljs-string">'mark'</span>));
  <span class="hljs-comment">/**
   * [
   *   PerformanceEntry {
   *     name: 'test',
   *     entryType: 'mark',
   *     startTime: 63.518931,
   *     duration: 0
   *   }
   * ]
   */</span>
  <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(perfObserverList.<span class="hljs-title function_">getEntriesByName</span>(<span class="hljs-string">'test'</span>, <span class="hljs-string">'measure'</span>)); <span class="hljs-comment">// []</span>
  observer.<span class="hljs-title function_">disconnect</span>();
});
obs.<span class="hljs-title function_">observe</span>({ <span class="hljs-attr">entryTypes</span>: [<span class="hljs-string">'mark'</span>, <span class="hljs-string">'measure'</span>], <span class="hljs-attr">buffered</span>: <span class="hljs-literal">true</span> });

performance.<span class="hljs-title function_">mark</span>(<span class="hljs-string">'test'</span>);
performance.<span class="hljs-title function_">mark</span>(<span class="hljs-string">'meow'</span>);</code></pre>
<h4><code>performanceObserverEntryList.getEntriesByType(type)</code><span><a class="mark" href="#perf_hooks_performanceobserverentrylist_getentriesbytype_type" id="perf_hooks_performanceobserverentrylist_getentriesbytype_type">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v8.5.0</span>
</div>
<ul>
<li><code>type</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type">&#x3C;string></a></li>
<li>Returns: <a href="perf_hooks.html#perf_hooks_class_performanceentry" class="type">&#x3C;PerformanceEntry[]></a></li>
</ul>
<p>Returns a list of <code>PerformanceEntry</code> objects in chronological order
with respect to <code>performanceEntry.startTime</code> whose <code>performanceEntry.entryType</code>
is equal to <code>type</code>.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> {
  performance,
  <span class="hljs-title class_">PerformanceObserver</span>
} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'perf_hooks'</span>);

<span class="hljs-keyword">const</span> obs = <span class="hljs-keyword">new</span> <span class="hljs-title class_">PerformanceObserver</span>(<span class="hljs-function">(<span class="hljs-params">perfObserverList, observer</span>) =></span> {
  <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(perfObserverList.<span class="hljs-title function_">getEntriesByType</span>(<span class="hljs-string">'mark'</span>));
  <span class="hljs-comment">/**
   * [
   *   PerformanceEntry {
   *     name: 'test',
   *     entryType: 'mark',
   *     startTime: 55.897834,
   *     duration: 0
   *   },
   *   PerformanceEntry {
   *     name: 'meow',
   *     entryType: 'mark',
   *     startTime: 56.350146,
   *     duration: 0
   *   }
   * ]
   */</span>
  observer.<span class="hljs-title function_">disconnect</span>();
});
obs.<span class="hljs-title function_">observe</span>({ <span class="hljs-attr">entryTypes</span>: [<span class="hljs-string">'mark'</span>], <span class="hljs-attr">buffered</span>: <span class="hljs-literal">true</span> });

performance.<span class="hljs-title function_">mark</span>(<span class="hljs-string">'test'</span>);
performance.<span class="hljs-title function_">mark</span>(<span class="hljs-string">'meow'</span>);</code></pre>
</section><section><h3><code>perf_hooks.createHistogram([options])</code><span><a class="mark" href="#perf_hooks_perf_hooks_createhistogram_options" id="perf_hooks_perf_hooks_createhistogram_options">#</a></span></h3>
<div class="api_metadata">
<span>Added in: v14.18.0</span>
</div>
<ul>
<li><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type">&#x3C;Object></a>
<ul>
<li><code>min</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt" class="type">&#x3C;bigint></a> The minimum recordable value. Must be an integer
value greater than 0. <strong>Defaults</strong>: <code>1</code>.</li>
<li><code>max</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt" class="type">&#x3C;bigint></a> The maximum recordable value. Must be an integer
value greater than <code>min</code>. <strong>Defaults</strong>: <code>Number.MAX_SAFE_INTEGER</code>.</li>
<li><code>figures</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a> The number of accuracy digits. Must be a number between
<code>1</code> and <code>5</code>. <strong>Defaults</strong>: <code>3</code>.</li>
</ul>
</li>
<li>Returns <a href="perf_hooks.html#perf_hooks_class_recordablehistogram_extends_histogram" class="type">&#x3C;RecordableHistogram></a></li>
</ul>
<p>Returns a <a href="perf_hooks.html#perf_hooks_class_recordablehistogram_extends_histogram" class="type">&#x3C;RecordableHistogram></a>.</p>
</section><section><h3><code>perf_hooks.monitorEventLoopDelay([options])</code><span><a class="mark" href="#perf_hooks_perf_hooks_monitoreventloopdelay_options" id="perf_hooks_perf_hooks_monitoreventloopdelay_options">#</a></span></h3>
<div class="api_metadata">
<span>Added in: v11.10.0</span>
</div>
<ul>
<li><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type">&#x3C;Object></a>
<ul>
<li><code>resolution</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a> The sampling rate in milliseconds. Must be greater
than zero. <strong>Default:</strong> <code>10</code>.</li>
</ul>
</li>
<li>Returns: <a href="perf_hooks.html#perf_hooks_class_intervalhistogram_extends_histogram" class="type">&#x3C;IntervalHistogram></a></li>
</ul>
<p><em>This property is an extension by Node.js. It is not available in Web browsers.</em></p>
<p>Creates an <code>IntervalHistogram</code> object that samples and reports the event loop
delay over time. The delays will be reported in nanoseconds.</p>
<p>Using a timer to detect approximate event loop delay works because the
execution of timers is tied specifically to the lifecycle of the libuv
event loop. That is, a delay in the loop will cause a delay in the execution
of the timer, and those delays are specifically what this API is intended to
detect.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> { monitorEventLoopDelay } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'perf_hooks'</span>);
<span class="hljs-keyword">const</span> h = <span class="hljs-title function_">monitorEventLoopDelay</span>({ <span class="hljs-attr">resolution</span>: <span class="hljs-number">20</span> });
h.<span class="hljs-title function_">enable</span>();
<span class="hljs-comment">// Do something.</span>
h.<span class="hljs-title function_">disable</span>();
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(h.<span class="hljs-property">min</span>);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(h.<span class="hljs-property">max</span>);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(h.<span class="hljs-property">mean</span>);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(h.<span class="hljs-property">stddev</span>);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(h.<span class="hljs-property">percentiles</span>);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(h.<span class="hljs-title function_">percentile</span>(<span class="hljs-number">50</span>));
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(h.<span class="hljs-title function_">percentile</span>(<span class="hljs-number">99</span>));</code></pre>
</section><section><h3>Class: <code>Histogram</code><span><a class="mark" href="#perf_hooks_class_histogram" id="perf_hooks_class_histogram">#</a></span></h3>
<div class="api_metadata">
<span>Added in: v11.10.0</span>
</div>
<h4><code>histogram.exceeds</code><span><a class="mark" href="#perf_hooks_histogram_exceeds" id="perf_hooks_histogram_exceeds">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v11.10.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
<p>The number of times the event loop delay exceeded the maximum 1 hour event
loop delay threshold.</p>
<h4><code>histogram.max</code><span><a class="mark" href="#perf_hooks_histogram_max" id="perf_hooks_histogram_max">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v11.10.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
<p>The maximum recorded event loop delay.</p>
<h4><code>histogram.mean</code><span><a class="mark" href="#perf_hooks_histogram_mean" id="perf_hooks_histogram_mean">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v11.10.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
<p>The mean of the recorded event loop delays.</p>
<h4><code>histogram.min</code><span><a class="mark" href="#perf_hooks_histogram_min" id="perf_hooks_histogram_min">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v11.10.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
<p>The minimum recorded event loop delay.</p>
<h4><code>histogram.percentile(percentile)</code><span><a class="mark" href="#perf_hooks_histogram_percentile_percentile" id="perf_hooks_histogram_percentile_percentile">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v11.10.0</span>
</div>
<ul>
<li><code>percentile</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a> A percentile value in the range (0, 100].</li>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
<p>Returns the value at the given percentile.</p>
<h4><code>histogram.percentiles</code><span><a class="mark" href="#perf_hooks_histogram_percentiles" id="perf_hooks_histogram_percentiles">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v11.10.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map" class="type">&#x3C;Map></a></li>
</ul>
<p>Returns a <code>Map</code> object detailing the accumulated percentile distribution.</p>
<h4><code>histogram.reset()</code><span><a class="mark" href="#perf_hooks_histogram_reset" id="perf_hooks_histogram_reset">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v11.10.0</span>
</div>
<p>Resets the collected histogram data.</p>
<h4><code>histogram.stddev</code><span><a class="mark" href="#perf_hooks_histogram_stddev" id="perf_hooks_histogram_stddev">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v11.10.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
<p>The standard deviation of the recorded event loop delays.</p>
</section><section><h3>Class: <code>IntervalHistogram extends Histogram</code><span><a class="mark" href="#perf_hooks_class_intervalhistogram_extends_histogram" id="perf_hooks_class_intervalhistogram_extends_histogram">#</a></span></h3>
<p>A <code>Histogram</code> that is periodically updated on a given interval.</p>
<h4><code>histogram.disable()</code><span><a class="mark" href="#perf_hooks_histogram_disable" id="perf_hooks_histogram_disable">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v11.10.0</span>
</div>
<ul>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type">&#x3C;boolean></a></li>
</ul>
<p>Disables the update interval timer. Returns <code>true</code> if the timer was
stopped, <code>false</code> if it was already stopped.</p>
<h4><code>histogram.enable()</code><span><a class="mark" href="#perf_hooks_histogram_enable" id="perf_hooks_histogram_enable">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v11.10.0</span>
</div>
<ul>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type">&#x3C;boolean></a></li>
</ul>
<p>Enables the update interval timer. Returns <code>true</code> if the timer was
started, <code>false</code> if it was already started.</p>
<h4>Cloning an <code>IntervalHistogram</code><span><a class="mark" href="#perf_hooks_cloning_an_intervalhistogram" id="perf_hooks_cloning_an_intervalhistogram">#</a></span></h4>
<p><a href="perf_hooks.html#perf_hooks_class_intervalhistogram_extends_histogram" class="type">&#x3C;IntervalHistogram></a> instances can be cloned via <a href="worker_threads.html#worker_threads_class_messageport" class="type">&#x3C;MessagePort></a>. On the receiving
end, the histogram is cloned as a plain <a href="perf_hooks.html#perf_hooks_class_histogram" class="type">&#x3C;Histogram></a> object that does not
implement the <code>enable()</code> and <code>disable()</code> methods.</p>
</section><section><h3>Class: <code>RecordableHistogram extends Histogram</code><span><a class="mark" href="#perf_hooks_class_recordablehistogram_extends_histogram" id="perf_hooks_class_recordablehistogram_extends_histogram">#</a></span></h3>
<div class="api_metadata">
<span>Added in: v14.18.0</span>
</div>
<h4><code>histogram.record(val)</code><span><a class="mark" href="#perf_hooks_histogram_record_val" id="perf_hooks_histogram_record_val">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v14.18.0</span>
</div>
<ul>
<li><code>val</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt" class="type">&#x3C;bigint></a> The amount to record in the histogram.</li>
</ul>
<h4><code>histogram.recordDelta()</code><span><a class="mark" href="#perf_hooks_histogram_recorddelta" id="perf_hooks_histogram_recorddelta">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v14.18.0</span>
</div>
<p>Calculates the amount of time (in nanoseconds) that has passed since the
previous call to <code>recordDelta()</code> and records that amount in the histogram.</p>
</section><section><h3>Examples<span><a class="mark" href="#perf_hooks_examples" id="perf_hooks_examples">#</a></span></h3>
<h4>Measuring the duration of async operations<span><a class="mark" href="#perf_hooks_measuring_the_duration_of_async_operations" id="perf_hooks_measuring_the_duration_of_async_operations">#</a></span></h4>
<p>The following example uses the <a href="async_hooks.html">Async Hooks</a> and Performance APIs to measure
the actual duration of a Timeout operation (including the amount of time it took
to execute the callback).</p>
<pre><code class="language-js"><span class="hljs-meta">'use strict'</span>;
<span class="hljs-keyword">const</span> async_hooks = <span class="hljs-built_in">require</span>(<span class="hljs-string">'async_hooks'</span>);
<span class="hljs-keyword">const</span> {
  performance,
  <span class="hljs-title class_">PerformanceObserver</span>
} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'perf_hooks'</span>);

<span class="hljs-keyword">const</span> set = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Set</span>();
<span class="hljs-keyword">const</span> hook = async_hooks.<span class="hljs-title function_">createHook</span>({
  <span class="hljs-title function_">init</span>(<span class="hljs-params">id, type</span>) {
    <span class="hljs-keyword">if</span> (type === <span class="hljs-string">'Timeout'</span>) {
      performance.<span class="hljs-title function_">mark</span>(<span class="hljs-string">`Timeout-<span class="hljs-subst">${id}</span>-Init`</span>);
      set.<span class="hljs-title function_">add</span>(id);
    }
  },
  <span class="hljs-title function_">destroy</span>(<span class="hljs-params">id</span>) {
    <span class="hljs-keyword">if</span> (set.<span class="hljs-title function_">has</span>(id)) {
      set.<span class="hljs-title function_">delete</span>(id);
      performance.<span class="hljs-title function_">mark</span>(<span class="hljs-string">`Timeout-<span class="hljs-subst">${id}</span>-Destroy`</span>);
      performance.<span class="hljs-title function_">measure</span>(<span class="hljs-string">`Timeout-<span class="hljs-subst">${id}</span>`</span>,
                          <span class="hljs-string">`Timeout-<span class="hljs-subst">${id}</span>-Init`</span>,
                          <span class="hljs-string">`Timeout-<span class="hljs-subst">${id}</span>-Destroy`</span>);
    }
  }
});
hook.<span class="hljs-title function_">enable</span>();

<span class="hljs-keyword">const</span> obs = <span class="hljs-keyword">new</span> <span class="hljs-title class_">PerformanceObserver</span>(<span class="hljs-function">(<span class="hljs-params">list, observer</span>) =></span> {
  <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(list.<span class="hljs-title function_">getEntries</span>()[<span class="hljs-number">0</span>]);
  performance.<span class="hljs-title function_">clearMarks</span>();
  observer.<span class="hljs-title function_">disconnect</span>();
});
obs.<span class="hljs-title function_">observe</span>({ <span class="hljs-attr">entryTypes</span>: [<span class="hljs-string">'measure'</span>], <span class="hljs-attr">buffered</span>: <span class="hljs-literal">true</span> });

<span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> {}, <span class="hljs-number">1000</span>);</code></pre>
<h4>Measuring how long it takes to load dependencies<span><a class="mark" href="#perf_hooks_measuring_how_long_it_takes_to_load_dependencies" id="perf_hooks_measuring_how_long_it_takes_to_load_dependencies">#</a></span></h4>
<p>The following example measures the duration of <code>require()</code> operations to load
dependencies:</p>
<!-- eslint-disable no-global-assign -->
<pre><code class="language-js"><span class="hljs-meta">'use strict'</span>;
<span class="hljs-keyword">const</span> {
  performance,
  <span class="hljs-title class_">PerformanceObserver</span>
} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'perf_hooks'</span>);
<span class="hljs-keyword">const</span> mod = <span class="hljs-built_in">require</span>(<span class="hljs-string">'module'</span>);

<span class="hljs-comment">// Monkey patch the require function</span>
mod.<span class="hljs-property">Module</span>.<span class="hljs-property"><span class="hljs-keyword">prototype</span></span>.<span class="hljs-property">require</span> =
  performance.<span class="hljs-title function_">timerify</span>(mod.<span class="hljs-property">Module</span>.<span class="hljs-property"><span class="hljs-keyword">prototype</span></span>.<span class="hljs-property">require</span>);
<span class="hljs-built_in">require</span> = performance.<span class="hljs-title function_">timerify</span>(<span class="hljs-built_in">require</span>);

<span class="hljs-comment">// Activate the observer</span>
<span class="hljs-keyword">const</span> obs = <span class="hljs-keyword">new</span> <span class="hljs-title class_">PerformanceObserver</span>(<span class="hljs-function">(<span class="hljs-params">list</span>) =></span> {
  <span class="hljs-keyword">const</span> entries = list.<span class="hljs-title function_">getEntries</span>();
  entries.<span class="hljs-title function_">forEach</span>(<span class="hljs-function">(<span class="hljs-params">entry</span>) =></span> {
    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">`require('<span class="hljs-subst">${entry[<span class="hljs-number">0</span>]}</span>')`</span>, entry.<span class="hljs-property">duration</span>);
  });
  obs.<span class="hljs-title function_">disconnect</span>();
});
obs.<span class="hljs-title function_">observe</span>({ <span class="hljs-attr">entryTypes</span>: [<span class="hljs-string">'function'</span>], <span class="hljs-attr">buffered</span>: <span class="hljs-literal">true</span> });

<span class="hljs-built_in">require</span>(<span class="hljs-string">'some-module'</span>);</code></pre></section>
        <!-- API END -->
      </div>
    </div>
  </div>
  <script>
    'use strict';
    {
      const kCustomPreference = 'customDarkTheme';
      const userSettings = sessionStorage.getItem(kCustomPreference);
      const themeToggleButton = document.getElementById('theme-toggle-btn');
      if (userSettings === null && window.matchMedia) {
        const mq = window.matchMedia('(prefers-color-scheme: dark)');
        if ('onchange' in mq) {
          function mqChangeListener(e) {
            document.body.classList.toggle('dark-mode', e.matches);
          }
          mq.addEventListener('change', mqChangeListener);
          if (themeToggleButton) {
            themeToggleButton.addEventListener('click', function() {
              mq.removeEventListener('change', mqChangeListener);
            }, { once: true });
          }
        }
        if (mq.matches) {
          document.body.classList.add('dark-mode');
        }
      } else if (userSettings === 'true') {
        document.body.classList.add('dark-mode');
      }
      if (themeToggleButton) {
        themeToggleButton.hidden = false;
        themeToggleButton.addEventListener('click', function() {
          sessionStorage.setItem(
            kCustomPreference,
            document.body.classList.toggle('dark-mode')
          );
        });
      }
    }
  </script>
</body>
</html>
